The present invention is an improvement in the infrared scanning system described and claimed in prior U.S. Pat. application Ser. No. 232,015, filed Mar. 6, 1972, now U.S. Pat. No. 3,798,366 issued Mar. 19, 1974. This prior application describes an existing thermograph product of the common assignee of the prior and this application, Spectrotherm Corporation of Santa Clara, Calif.
Such a thermograph instrument utilizes electromechanical optics to scan a two dimensional object (most commonly a human patient under medical diagnosis) across a point infrared sensitive detector. The detector output is a single time varying electrical signal proportional to the infrared intensity variations of the object as it is scanned line by line in a two dimensional raster pattern analogous to that utilized in typical television systems. This electrical signal is then processed and applied to a video monitor. A visual picture is thus reproduced from the processed electronic signal on the face of the video monitor showing light and dark intensity variations of an image of the object corresponding to high and low temperature areas, respectively, of the object being imaged. Such a thermograph device has become widely accepted as a medical diagnostic tool for, among other applications, detecting the existence of cancerous growth below the skin of human patients.
In order to reduce the bandwidth capability required of the video processing circuits and to prevent horizontal shading effects, a reference target is provided within the thermograph that is imaged thereby at the beginning of each line scan across the object. At the same time that the reference target is being imaged, the signal level in the video processing circuits is referenced to a particular voltage, namely ground potential in a specific thermograph instrument. This technique eliminates the necessity of carrying the electronic bandwidth to a very low frequency and serves to overcome some errors introduced into the system by drifting of components, charge accummulation, etc.
This technique is satisfactory to assure that the optical images remain stable for a short period of time but over a longer period of use of the thermograph the temperature of the reference target is likely to slowly rise. Thus, the brightness of the optical image formed on the video monitor for a given object temperature will change over time as the reference target temperature changes. To overcome this, an automatic brightness control is provided as part of the video processing circuitry in the existing thermograph identified above. Such a circuit references the brightness of the reconstructed image points to the brightest point of the object being scanned. This technique is satisfactory in many circumstances but has a disadvantage in others that the cool features of the object may be lost in the very dark portions of the optical image. Furthermore, no absolute comparison of object temperatures is possible from the optical reconstructions themselves since each optical image of a different object will be referenced to a different brightness level depending on the characteristics of that object.
Another thermograph instrument has solved this problem by maintaining its reference target at a constant temperature no matter what is happening to the temperature of the environment or that within the thermograph instrument. Each video picture reconstructed is thus referenced to a common reference temperature detected from the reference target. However, the maintenance of the target at a constant temperature is extremely cumbersome and complex.
Therefore, it is a primary object of the present invention to provide an improved technique for absolute temperature reference in an infrared imaging instrument.